The present invention is generally concerned with cellular mobile radio systems, i.e. cellular mobile radio networks and mobile stations for use in such networks.
The present invention is more particularly concerned with selecting a cell when a mobile station accesses a network of the above kind to make a call.
The mobile station itself selecting a cell for access to the network is known per se, the corresponding algorithm used in the mobile station being known as a cell selection algorithm.
To be more precise the network broadcasts in the server cell, on a particular broadcast control channel, information needed to execute an algorithm of the above kind and including a list of neighboring cells for the mobile station to monitor to determine whether the signal level received on the broadcast control channel from the neighboring cells or from the server cell is more favorable. Providing that certain other criteria are satisfied, the cell determined in this way is selected for access to the network. What is more, when the mobile station is first put into service, at which time no server cell has been selected, the mobile station scans all the broadcast control channels and determines the server cell using the same criteria.
For a more complete description of cellular mobile radio systems such as the Global System for Mobile communications (GSM) see xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by M. MOULY and M. B. PAUTHET (Palaiseau, France, 1992, ISBN:2-9507190-7-7).
The present invention is even more particularly concerned with cell selection during access to a cellular mobile radio network for making a call when the network includes different types of cell in which different types of service are available.
One example of a network of the above kind is a network in which new means of radio access to that infrastructure, corresponding to a system such as a third generation system (in particular the Universal Mobile Telecommunication System (UMTS)), are successively introduced into an existing infrastructure corresponding to a system such as a second generation system (in particular the previously mentioned GSM) in order to introduce new services progressively, in particular high bit rate data transmission services.
A system of the above kind thus includes cells or resources in which new services are available, in this instance UMTS cells or GSM/UMTS cells (i.e. cells having a broadcast control channel common to GSM type radio access means and to UMTS type radio access means), and cells in which said new services are not available, in this instance GSM cells.
In a system of the above kind the cell selection algorithm mentioned above may select a GSM cell although the service required for a given call for which the mobile station is accessing the network would require a UMTS or GSM/UMTS cell, or conversely it may select a UMTS or GSM/UMTS cell although a GSM cell would suffice for the service required, with the result that in neither case is radio resource allocation optimized to suit the services required by users in a network of the above kind.
Another example of a network of the above kind is a network in which additional radio access means dedicated to a particular user group and enabling those users to obtain additional services in a given geographical area are introduced into an existing infrastructure corresponding to a given system, for example the GSM, and that geographical area.
In a system of the above kind the cell selection algorithm previously mentioned may select a cell in which said additional services are not available, in particular if a user authorized to use the additional services is at the limit of the geographical area, in which case neither the user nor the provider of the services is satisfied.
Thus the problem of the cell selected for access to the network not necessarily corresponding to a better cell in terms of services generally arises in a cellular mobile radio system including different types of cells in which different types of services are available.
In a cellular mobile radio network such as the GSM network, selecting a cell during a call to determine a better cell to which the call is to be handed over is known per se. Having the network select the cell on the basis of measurement results transmitted to it by the mobile station is also known per se. To be more precise, the cell selected is the first from a list of candidate cells listed in an order of preference according to the measurement results and to which handover is possible, i.e. in particular in which resources are available for the call.
In a cellular mobile radio network such as the GSM network xe2x80x9cdirected retryxe2x80x9d, i.e. handover from a cell initially selected for access to the network to a better cell selected subsequently, is known per se, and in this case it may be necessary to allow for the fact that the cell is selected differently in the two situations, as just outlined, which can lead to different cells being selected.
Document W097/44922 describes a cellular mobile radio system including different types of cells in which different types of multimedia service are available and in which a best cell to which to hand over a call in progress is selected on the basis of measurement results for candidate cells, the availability of resources in those candidate cells and the type of service requested. However, the above document does not cover cell selection on accessing the network to make a call. Also, the above document describes a mobile station provided with means enabling it to select the best cell to which to hand over a call in progress. If such cell selection were implemented in the mobile station to select a cell when accessing the network to make a call, this would have the drawback of necessitating many exchanges of signaling between the mobile station and the network, leading to high consumption of radio resources for such exchanges, and thus to an increase in the overall level of interference in the network. The present invention also aims to avoid these drawbacks.
Thus in particular the invention aims to enable access by a mobile station a cellular mobile radio network including different types of cell in which different types of service are available in a way that avoids the various drawbacks mentioned above.
The present invention therefore consists in a method of selecting a cell when a mobile station accesses a cellular mobile radio network to make a call, said network including different types of cells in which different types of services are available, said method including the following steps:
selection by said mobile station of a cell to access the network,
access to the network in the cell so selected, corresponding to a given type of cell, and
deciding whether or not to hand over said call to a cell of another type constituting a better cell in terms of services, depending on the type of service required for said call.
In other words, the invention can be considered as introducing a new application of the xe2x80x9cdirected retryxe2x80x9d concept whereby a call is handed over from a cell initially selected for access to the network to another cell constituting a better cell in terms of service.
The present invention also consists in a cellular mobile radio network enabling the use of a method of the above kind (the present invention having the advantage of not requiring any modification of the mobile stations to enable it to be implemented).